Large angle titled implant ("LATI") of either pocket, halo or source/drain extensions is currently needed in ultra large scale integrated ("ULSI") complimentary metal oxide semiconductor ("CMOS") transistors in order to suppress "short-channel effects." Short channel effects are described in further detail in application Ser. No. 09/122,815, filed on Jul. 27, 1998, entitled MOS TRANSISTOR WITH HIGH-K SPACER DESIGNED FOR ULTRA-LARGE SCALE INTEGRATION, commonly assigned to the assignee of record for the present application, the contents of which are incorporated herein by reference.
Limitations exist, however, of applying LATI techniques in ULSI circuits. For example, due to the very high packing density of a ULSI circuit, shadowing effects from both the gate stack and photo-resist layers restricts the maximum tilt angle that can be used for ion implantation. Presently, the largest tilt angle used commercially for implants is illustrated schematically in FIG. 1. As shown in FIG. 1, the two angles .theta..sub.1, .theta..sub.2 corresponding to the shadowing effect from gate stacks and photo-resist layers, respectively, can be expressed according to the following equation: EQU .theta..sub.MAX =min(.theta..sub.1.theta..sub.2)
As can be seen, the maximum tilt angle, .theta..sub.max, is dependent upon the gate stack height A and the thickness of the photo-resist layer B. This maximum angle of implantation, however, is smaller than what a device engineer currently may desire in the fabrication of ULSI circuits, which is typically larger than 40.degree..
Another limitation of applying LATI techniques in ULSI circuits is that the largest angle allowable for implants is impacted by the random variation of several process parameters. These parameters include for example gate critical dimensions, the thickness of a gate stack and the thickness of the photo-resist layer. Lithography overlay may also have a significant impact. In a worst case scenario of lithography overlay, therefore, the available .theta..sub.MAX could ultimately be very small.
What is lacking in the art is a small angle tilted implant technique that has the equivalent doping effect of large angle tilted implants. One advantage of the small angle tilted implant technique over the large angle tilted implant technique is the circumvention of the maximum angle limit (.theta..sub.MAX) that occurs in the large angle method. Another advantage to a small angle tilted implant technique would be self alignment of the pocket/halo/extension implant to the gate of the device.